1. Field of the Invention
The present invention relates to a small-sized fuel cell system for use in a portable device and a control method thereof.
2. Description of the Related Art
Direct methanol fuel cells (DMFCs) have been under development as small power sources for portable devices for the following reasons: auxiliary equipment such as a vaporizer and a humidifier is not required; methanol is easier to handle than gas fuels such as hydrogen; and operation at low temperature is possible.
The DMFC comprises a membrane-electrode assembly (MEA), an anode and a cathode, and for the DMFCs, there are an active type using the auxiliary equipment for fuel supply to the anode and the cathode, and a passive type utilizing natural force. In the former active DMFC, the output density of a cell can be increased, but auxiliary equipment is essential, such as a pump for fuel supply to the anode and cathode.
Furthermore, in the DMFC, water of the same mol as that of methanol is required for an anode reaction. Moreover, when highly concentrated methanol is directly supplied to the MEA, methanol crossover is increased, and the efficiency of fuel use thus decreases. Therefore, a water collection mechanism can be provided as auxiliary equipment on the cathode side in order to efficiently collect water produced in a cathode reaction and mix the collected water with the highly concentrated methanol fuel in a mixing tank. It is to be noted that another possibility is a system in which the fuel is only circulated on the anode side without collecting water on the cathode side in order to prevent an increase in the volume of an apparatus due to the installation of the water collection mechanism on the cathode side.
In such a circulation-type fuel cell system, it is important to maintain a proper concentration of the fuel supplied to the anode and to maintain a proper volume of liquid in the mixing tank for sending the fuel to the anode. Various proposals have heretofore been made for systems to control the concentration and volume of the fuel. For example, JPA 2005-32610(KOKAI) incorporated by reference proposes a system in which the flow volume of a fuel from a highly concentrated fuel tank and a cathode water collection unit are manipulated to control the concentration of a mixture. Further, JPA 2005-11633 (KOKAI) incorporated by reference proposes a system in which the amount of a fuel supplied from a highly concentrated fuel tank and a cathode side water collection amount are manipulated to control the concentration of a mixture. Still further, JPA 5-258760 (KOKAI) incorporated by reference proposes a system in which the amount of a fuel supplied from a highly concentrated fuel tank and the amount of water supplied from a water tank are manipulated to control the volume and concentration of a mixture. Further yet, JPA 2005-108713 (KOKAI) incorporated by reference proposes a system in which air discharged from a cathode is sent into a condensation unit, and a condensation fan and a water collection unit are manipulated to change the amount of water supplied to a mixing tank, thereby controlling the volume and concentration of a mixture.
However, to control the concentration and volume of the fuel in the mixing tank, each of these conventional fuel cell systems comprises a cathode side water collection unit, an anode side fuel supply unit, a water collection/circulation path connecting the cathode side with the anode side, and a water tank to supply water from the outside. The volume occupied by the auxiliary equipment including the water collection unit, the water collection/circulation path and the water tank is large, which increases the size of the entire apparatus. Thus, the conventional fuel cell systems are too large to be used as power sources for portable small devices such as mobile telephones, portable audio terminals and notebook computers.
On the other hand, if the water tank, the water collection/circulation path, etc. are eliminated from the fuel cell system to reduce the size of the system, the efficiency of fuel use decreases.